Many structures have been proposed for the manufacture of inductors or transformers in integrated circuits. Many structures comprise a planar, spiral arrangement of conductive track, arranged in a plane parallel to the semiconductor substrate. By adding several of these in magnetic communication, a transformer is formed.
FIG. 1 shows such a transformer, composed of multiple planar spiral inductors, as described by Burghartz et al. in IEEE Transactions on Electron Devices, vol. 43, No 9, September 1996. Upon a substrate 6, a spiral inductor is formed in each of several conductive layers 10, 12, 14, 16, respectively separated by insulating layers 18, 20, 22. Electrical contact is provided to each end 24, 26 of each spiral inductor. Due to magnetic coupling between the spirals, an alternating signal applied to one spiral will be detectable on some or all of the other spirals, thus performing a transformer function.
Such a transformer has at least three major drawbacks. Firstly, the electromagnetic coupling between spiral layers is poor, since the distances between consecutive spirals 10, 12 is defined by the thickness of an interlevel dielectric layer 18. These interlevel dielectric layers tend to be designed to be thick enough to avoid parasitic capacitance between conductors of consecutive conductive layers, elsewhere in the integrated circuit. They may typically have a thickness of 1 .mu.m.
There will therefore be a low coupling coefficient between the spirals 10, 12 and the factor of merit of the resulting transformer, defined as the ratio of the current-voltage product of an input signal on a primary winding to the current-voltage product of an output signal from a secondary winding, is low.
Secondly, the electromagnetic field generated by the spiral inductors is substantially vertical; perpendicular to the surface of the substrate 6. During operation, a substantial amount of electromagnetic field will penetrate into the substrate 6. This is disadvantageous because a voltage is thereby induced in the substrate. Such voltage may cause leakage currents; may turn on some components within the integrated circuit; and may modify the operating characteristics of components of the integrated circuit, so that the integrated circuit is rendered inoperative.
Alternatively, a ground plane could be placed on the lowest conductive layer 10. This would, however, increase the parasitic capacitance associated with the structure, reducing the factor of merit of the resulting transformer. The introduction of a conductive plate may also have undesirable antenna effects.
Thirdly, the illustrated transformer occupies a large surface area of the substrate 6. The cited document discloses an area of 226 .mu.m.times.226 .mu.m for a spiral inductor of 3-6 turns, and an inductance of 1.4-11 nH.
FIG. 2 shows another prior art transformer for integrated circuits, as published in European Patent Application EP-A-0 725 407. Using two conductive layers 10, 12, and a series of vias 30, two concentric, intertwined windings A-A', B-B'are provided, wound around a ferromagnetic core 31. This transformer provides an increased coupling between the primary winding A-A' and the secondary winding B-B'. The substrate surface area occupied by this structure is still relatively large. The surface area will be defined by the necessary size of the ferromagnetic core, the required spacing between that and the vias 30 of the windings and the size of the vias, in the width direction, and the spacing between adjacent metal tracks and the required number of turns, in the length direction. For each turn of the primary and secondary windings, two windings are made around the core 31, meaning that the resulting transformer is twice as long as would be a simple inductor with the same number of turns as one winding of the transformer.
The use of ferromagnetic materials, such as required to fabricate the core 31 is not well controlled in current integrated circuit manufacturing techniques. Furthermore, the coupling between the windings is not very high, although improved with respect to the structure of FIG. 1.